Complete group

A group $G$ whose centre is trivial (that is, $G$ is a so-called group without centre) and for which all automorphisms are inner (see Inner automorphism). The automorphism group of a complete group $G$ is isomorphic to $G$ itself (the term "complete" is related to this property). Examples of complete groups are the symmetric groups $S_n$ when $N \ne 2,6$. If a group $G$ contains a normal subgroup $B$ which is complete, then $G$ decomposes into a direct product $B \times K$ of the subgroup $B$ and its centralizer $K$ in $G$; indeed $K$ is isomorphic to the quotient group $G/B$.

References

[1]	M.I. Kargapolov, Yu.I. Merzlyakov, "Fundamentals of group theory" , Moscow (1982) (In Russian)
[2]	M. Hall jr., "Group theory" , Chelsea (1976)

[a1]	William Burnside, "Theory of Groups of Finite Order", 1911 ed. repr. Cambridge University Press (2012) ISBN 1108050328